Qubits and quantum devices
Can a diamond quantum sensor take high-resolution NMR of tiny volumes?
Open access · cc by · source: Europe PMC
Separating the magnetising step from the diamond-based detection step let researchers record NMR spectra ten times sharper than earlier diamond sensors, enough for two-dimensional NMR on picolitre volumes.
Study at a glance
- Design
- Other — Lab experiment: analyte prepolarised in a 1.5 T magnet, flowed to a 13 mT detection region and read out by NV-centre ensembles in a diamond membrane inside a microfluidic chip.
- N
- No sample N; measurements on water, trimethyl phosphate and 1,4-difluorobenzene with several diamond membranes.
- Population
- Liquid analytes (water, TMP, DFB) sensed by NV-centre ensembles in 35-micrometre diamond membranes
- Outcome
- NMR linewidth (spectral resolution), concentration sensitivity, J-coupling splittings and 2D COSY cross peaks
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Key findings
The narrowest water line was 0.65 Hz wide, about an order of magnitude sharper than previous diamond NMR, and concentration sensitivity was about 27 M s^1/2. Roughly 80% of the prepolarised magnetisation survived transfer to the detector. Spectra resolved heteronuclear J-coupling splittings of 11.04 Hz in trimethyl phosphate and about 6 Hz in difluorobenzene, and a heteronuclear COSY experiment showed cross peaks demonstrating magnetisation transfer mediated by those couplings.
Methodology
Liquids were first magnetised by flowing through a strong permanent-magnet array, then moved to a weak, carefully stabilised field where they sat over a diamond membrane containing nitrogen-vacancy centres. Laser and microwave pulse sequences let the NV centres sense the tiny oscillating magnetic field of precessing proton spins in an effective volume of about 40 picolitres. The team calibrated sensitivity with a known test field, measured water, trimethyl phosphate and difluorobenzene, and ran two versions of 2D correlation spectroscopy.
Limitations
The low detection field of 13 mT means chemical shifts cannot be resolved, so the method cannot yet identify molecules the way high-field NMR does. Sensitivity is far too low for metabolites at physiological concentrations without long averaging, and it varied by about 50% between experiments. Although the sensed volume is picolitres, several millilitres of liquid are needed to fill the flow apparatus, and the linewidth is still wider than water's natural value.
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